SUBSTITUENT EFFECTS ON THE GENERAL ACID-CATALYZED AND SPECIFIC ACID-CATALYZED HYDROLYSES OF ACETALS OF BENZALDEHYDES

The effects of solvent and substituents on the general-acid-catalyzed (GAC) hydrolysis of acetals of benzaldehydes are defined. Contrary to earlier reports, GAC hydrolysis is important for a broad spectrum of benzaldehyde acetals. A variety of linear free energy relationship (LFER) cross correlations are established so that the Brønsted α may be calculated for nearly any acetal of any benzaldehyde. The smallest a measured is 0.70 for the hydrolysis of benzaldehyde trifluoroethyl ethyl acetal and the largest is defined as >0.9 for the hydrolysis of m-chlorobenzaldehyde diethyl acetal. Procedures for accurately measuring large α values are established. These results reemphasize the importance of defining α as opposed to use of the more ambiguous term specific acid catalysis. The mechanistic implications of LFER cross correlations are discussed in terms of Jencks-ƠFerrall diagrams. Evidence is summarized for rate-determining diffusional separation of the (carboxonium ion)•(conjugate base of the catalyzing acid)•(alcohol) aggregate. © 1979, American Chemical Society. All rights reserved.